The trimming coils embedded in poles of magnet are applied in the C-dipole prototypes of CSR. They are used to adjust the uniformity of magnetic fields through reducing outside magnetic field and augmenting inside. The impact of trimming currents on the uniformity of field and multipole components is carefully investigated by using the two kinds of magnetic measurements, hall sensor and long coil, respectively. It is proved that the uniformity which is better than +/-1.0x10(-3) can be reached easily. At the same time, the quadrupole and sextupole components of fields have decreased to a low level through optimizing the trimming currents, but, unfortunately, the octupole and dodecapole will increase correspondingly. Finally,the two measured and calculated results are compared. There is very small difference between the results of the two measuring methods. However,all of the measured results are alien to the calculated one because of the insufficiency of the TOSCA calculation.